1. Field of the Invention
The invention pertains to acoustical well logging and more particularly to treating well-logging signals so as to more accurately determine the timed first arrival of the acoustic signal after the transmitter burst by measuring energy content of the signal relative to threshold noise.
2. Description of the Prior Art
Conventionally, the received analog logging signal is detected at the surface from the cable connected to the downhole logging tool and amplified in a manually controlled amplifier. The output of the amplifier is applied to time measuring circuits where the zero-crossing location of the signal after the arrival of the signal greater than a preset threshold level, is used as the time reference point for the onset of the compression wave. As mentioned, it is conventional that the gain of the amplifier be log-operator dependent. Further, it is common to make one logging run while setting the amplifier and then make another run at the amplifier setting for developing the actual logging signal to be utilized in analysis.
The detection of the first arrival of the acoustic wave or signal in downhole receivers is difficult for several reasons. These reasons include absorption of energy by soft formations, widely spaced receivers, mechanical noise as from tool movement and electrical noise produced from electronics within the tool. Energy absorption is a significant problem since it reduces the amplitude of the first few cycles in the acoustic wave arriving at each receiver. The conventional amplitude discriminator and method gives poor results under adverse signal conditions, i.e., low signal levels and high noise levels.
Therefore, it is a feature of the present invention to provide an improved method and apparatus for treating well logging signals so as to accurately determine the first arrival of the acoustic wave at a downhole receiver after the transmitter burst by measuring the energy content of the signal relative to threshold noise.
It is another feature of the present invention to provide an improved method and apparatus that provides a pulse signal of a given magnitude and duration indicative of the first arrival of the acoustic wave at a downhole receiver.
It is yet another feature of the present invention to provide an improved method and apparatus that accurately determines the first arrival of the acoustic wave at a downhole receiver by producing a pulse of given amplitude and duration irrespective of threshold noise or subsequent acoustic wave arrivals.
It is an additional feature of the present invention to provide an improved method and apparatus that accurately detects by a pulse the first arrival of the acoustic wave at a downhole receiver with greatly improved detection ability and repeatability while the electronic circuitry required for this result has been reduced in comparison to the conventional amplitude discriminator systems.